1. Field of the Invention
The present invention relates to a circuit for discriminating a dot image area from digital image data of a document read by a facsimile apparatus or the like.
2. Description of the Prior Art
Most prints of a half-tone image and of a color image available today include a dot image photograph. A dot image is composed of dots arranged regularly and usually it has various spatial frequencies . When such a dot image is read with an image sensor in a digital copying machine, a facsimile apparatus or the like, if the fundamental and a higher harmonic frequency of the dots in a dot image is similar to the sampling frequency of an image sensor used for reading the dot image, a beat of a low frequency happens and it can be detected with naked eyes. This is a sampling Moire pattern which degrades the image quality. Further, especially for a facsimile apparatus, if the frequency of the dots is higher than the sampling frequency, the gradation of the read image data may be changed largely each pixel and this decreases the compression ratio remarkably when an ordinary coding method suitable for a character image is used.
In order to solve this problem, high frequency components have to be reduced by smoothing the dot image data. However, for an image composed of both a dot image and a character image, the smoothing is performed even on the character image. Thus, the resolution in the character image area decreases to degrade the image quality.
Therefore, a method was proposed to discriminate a dot image area from a character image area in order to change the processing according to the discrimination of a dot image area and a character image area. For example, in an apparatus disclosed in Japanese Patent laid open Publication No. 194,968/1986, the number of change points, at which the density level changes largely, is counted in successive pixels in a block of pixels both in the main scan direction and in the subscan direction, and a dot image area is discriminated according to the sum of the count values in each block.
However, this method needs two independent counters for counting change points both in the main scan direction and in the subscan direction, in order to discriminate dots from characters having lines extending in the subscan direction correctly. Therefore, it is difficult to process at a fast rate and to reduce the cost simultaneously.